The typical papermaking machine incorporates a number of vacuum pumps for creating a vacuum or subatmospheric pressure in various suction devices, such as suction boxes and couch rolls, used to extract water from the wet paper web. Water seal vacuum pumps are commonly used, as opposed to centrifugal pumps, and depending upon its location of use, each vacuum pump may be required to generate a high vacuum up to about 24 inches of mercury, or a lower vacuum down to 3 inches of mercury. The typical water seal vacuum pump uses substantial quantities of sealing water up to perhaps 100 gallons per minute. As a typical papermaking machine may incorporate from five to seven vacuum pumps, the pumps can utilize up to 700 gallons of sealing water per minute.
The sealing water is heated by the mechanical action of the pump itself, as well as by condensation of water vapor which is drawn from the suction box or couch roll. It is important to prevent the sealing water from being heated to a temperature that could cause vaporization, and thus the common practice has been to cool the sealing water by passing the water through a cooling tower and then recirculating it to the vacuum pump. In the cooling tower some evaporization occurs, which thereby increases the concentration of dissolved inorganic salts and additional makeup water is thus required. Due to the increase in concentration of inorganic salts, it has been the practice to utilize a "blow down" in which about 10% of the water is continuously discharged and additional makeup water is added. The specific amount of water discharged in the "blow down" depends on the salt concentration of the makeup water.
With the advent of small papermaking mills using recycled paper, it is desirable to locate such mills in urban sites, in close proximity to both available waste paper supply and the key markets for paper. With an urban location there is a critical need to have zero water discharge from the papermaking system, so as to minimize environmental impact.
To facilitate low energy costs, papermaking mills are preferably situated in close proximity to power plants and low pressure steam from the power plant turbine exhaust can be supplied to the dryer drums of the dryer section of the papermaking machine to dry the paper. The steam condensate is discharged from the dryer drums to a condensate receiver tank and the condensate can either be returned to the power plant or discharged to the sewer. While the steam condensate, or other condensate from evaporation systems, contains only a fraction of the ionic content of water from a typical municipal system, it can be contaminated with iron oxide or other trace impurities. Since power is normally generated in high pressure turbines, it is critical that the water being supplied to the turbines be extremely low in all contamination. As the dryer drums in the dryer section of the papermaking machine are basically rotating pressure vessels with rotary seals for steam and condensate, the condensate can, at times, become contaminated due to seal leakage. Therefore in many cases the steam condensate from the dryer drums cannot be returned to the power plant for use as makeup water. Because of this, it has been the common practice to discharge the condensate from the dryer drums to the sewer or waste system.